There are several proposals to 3GPP2 for OFDMA VoIP implementations, one of which defines numerology such that an OFDMA resource consisting of a set of 340 sub-carriers in frequency over OFDM symbol durations in time is divided into 20 ms VoIP frames, each containing 24 slots, each slot containing 10 OFDM symbols. The resources of each slot are further subdivided into distributed resource channels (DRCH), each comprising 81 subcarrier locations distributed across the 10 symbols of a slot for a total of 40 DRCHs per slot allowing for pilots and other overhead that might be present.
Transmission for a given user occurs at different rates or frame sizes. For example, the EVRC (Enhanced Variable Rate Codec) codec generates voice frames with four different rates or frame sizes: full, 1/2, 1/4 and 1/8 with probabilities of 29%, 4%, 7% and 60% respectively. The particular rate is typically determined as a function of a voice activity factor.
For a given user, a single packet is nominally expected to be delivered within one VoIP frame. Current definitions allow for an initial attempt to deliver the packet and three subsequent attempts. Any attempt, including the initial or subsequent, is referred to herein as a sub-packet.
A few variations of H-ARQ transmission/operation schemes exist. One variation is unicast H-ARQ in which each encoded packet includes data from one user. This can be fully asynchronous in which case the modulation and code rate (MCS (modulation and coding scheme), transmission time (slot/frame) and resource allocation are independent for each transmission of an encoded packet (first and all re-transmissions). Assignment signalling is used to describe the resource allocation, MCS and user IDs for each transmission and re-transmission. While this approach allows adaptation to real time channel conditions, it incurs large signalling overhead. Unicast H-ARQ can alternatively be fully synchronous. In this case, the MCS scheme for transmissions (first and all retransmissions) is the same, resource allocation (location) remains the same for first and all retransmissions (the transmission location must be the same as the first transmission). The transmission interval is fixed, and assignment signalling is required only for the first transmission. This enables lower signalling overhead for retransmission, but can cause significant scheduling complexity and signalling overhead for the first transmission due to the irregular vacancies of resources that occurs since some resources need to be reserved for retransmissions that may not be necessary.
Another H-ARQ variant is multicast H-ARQ in which each encoded packet includes data for multiple users. The worst CQIs (channel quality indicators) among multiple users are considered for selecting MCS. The entire packet is retransmitted if one or more users could not decode it successfully, even though some of the users may have successfully decoded the packet. Multi-cast H-ARQ can be implemented using fully asynchronous and fully synchronous schemes.